Prove that if x is a rational number and y is an irrational number,then x+y is an irrational number.

mohanty59supriya

mohanty59supriya

Answered question

2022-07-16

Prove that if x is a rational number and y is an irrational number,then x+y is an irrational number. If in addition, x≠0, then show that xy is an irrational number.

Answer & Explanation

Jeffrey Jordon

Jeffrey Jordon

Expert2022-11-07Added 2605 answers

Hint: Consider (x+y)x

Explanation:

As is very often the case, we do not need to write this as a proof by contradiction. We can prove the contrapositive directly.

We can prove directly:

x is rational  (x+y is rational  y is rational)

(using a,bQabQ -- that is, Q is closed under subtraction)

Therefore (by contraposition of the imbedded conditional)

x is rational  (y is not rational  x+y is not rational)

This is logically equivalent to

(x is rational & y is not rational)  x+y is not rational)

By contradiction

Suppose p and ¬q and r .

Prove a contradiction and conclude that if p and ¬q, then ¬r

By contrapositive

Suppose p and ¬r. Prove that q.

Conclude that If p and ¬q, then r.

The two methods are very closely related and I don't know of anyone who accepts one and not the other. (Although many/most/all intuitionists refuse to accept either contradiction or contrapositive.)

Proof by contrapositive

Suppose that x is rational and x+y is rational.

Then the difference (x+y)x=y is rational.

Hence is we know that y is irrational, then x+y must have been irrational.

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